In conventional radio communication devices which employ a composite diversity reception system that radio-wave signals modulated by a quadrature phase shift keying(QPSK) manner are received by a plurality of antennas and composed, means for converting phase difference angle data into trigonometric function values is used to compute the phase difference data of received radio wave by converting from a polar coordinates system to a rectangular coordinates system.
Conventionally, to compute a trigonometric function value from angle data, a method of computing the trigonometric function value by CPU is used for unpredictable inputs. On the other hand, typically used for predictable inputs is a conversion manner that calculation results are registered in a ROM table etc. and a calculation result written therein is accessed and output according to an input address related to an angle data.
FIG.1 shows an example of such a conversion manner, where sine values are stored corresponding to angles in a sine-wave table and cosine values are stored corresponding to angles in a cosine-wave table. When an angle data is input, a sine value and a cosine value are output corresponding to an address related to the angle data.
Japanese patent application laid-open No.7-307724(1995) discloses a diversity device which employs a like conversion manner that, when composite diversity is conducted, a trigonometric function value is calculated from a value previously stored in ROM after a phase difference of input wave is calculated.
However, it is not desirable that the method of computing the trigonometric function value by CPU is used for predictable inputs, since the manufacturing cost of the device is increased and the scale of the circuit is enlarged. On the other hand, the conversion manner with a storage medium like ROM is suitable for providing a low-cost device and a smaller circuit. However, a large storage medium is required when the angle data to be input is fractionized to improve the precision. For example, when it is fractionized to one degree, a storage capacity of 360 words is necessary.